This invention comprises clutch drive units, more particularly clutching units of the type which are spring engaged and electromagnetically disengaged so as to be fail safe, that is, in the event of electrical failure the drive through the unit is maintained. Such clutches are highly desirable in the context of automotive fan drives since, if there is an electrical failure in the power circuit to the clutch, it would be highly undesirable if the vehicle were disabled by such electrical failure.
Such electromagnetic operators for clutch units typically comprise an inner body and an armature which are brought into and out of engagement under the influence of the electromagnetic field set up by an electromagnetic coil. The armature is maintained in the axially spaced apart position by means of springs with an air gap between the armature and the inner body pole faces.
The magnetic flux passes through such gap and draws the armature and inner body axially together. The flux path generally comprises ferromagnetic materials except for the air gap which it must cross upon energization of the coil, and thus, this air gap comprises a substantial proportion of the reluctance of the overall magnetic flux path.
In the aforementioned type of clutch, the clutching surfaces, when engaged, relatively position the armature and inner body, and thus, if the clutching surfaces are subject to significant wear, the axial position of the inner body and armature would increase within increasing degrees of wear, increasing the air gap which must be crossed by the magnetic flux. The electromagnetic coil should, therefore, be sized to provide a sufficiently strong magnetic field to pass through the maximum air gap which may develop over the service life of the clutch and still reliably axially attract the armature and inner body into engagement. This necessitates the sizing of the electromagnetic coil to be considerably larger than it otherwise need be in order to account for the increasing air gap due to wear.
In U.S. Pat. No. 4,030,583 there is disclosed an arrangement for substantially overcoming the problems associated with increasing air gap with clutching surface wear. In that arrangement, a wear compensation feature is provided by providing a separate outer body rotatably secured to the inner body, but axially movable relative thereto. The outer body is provided with the first clutching surface engagable with the output clutching surface. The outer body is also threadably connected to the armature with spring operating means acting on the outer body to urge the outer body into clutching engagement. The armature is provided with an abutment surface which comes into engagement with the output member if the clutching surfaces have become worn. The abutment establishes the maximum air gap which may develop.
Upon the armature coming into abutment with the surface on the output member, it is driven relative to the outer body, which driving creates a relative axial movement therebetween tending to advance the outer body into clutching engagement with the output member clutching surface until the maximum torque is developed in the unit. Thereafter, the clutching surfaces engage with normal cycling of the clutch unit until enough wear has occurred to cause another cycle of wear compensation adjustment. Since the outer body and the armature are connected only through the threaded connection, relative movement of the outer body and armature could occur upon normal clutch engagement tending to defeat the wear compensation and cause excessive wear at the armature surfaces. Thus, in that arrangement a separate retarder is provided, which insures unidirectional movement of the threaded connection only in the direction tending to offset wear.
The separate retarder adds expense to the unit and it would be advantageous if the need for such a retarder could be eliminated without comprising either the effectiveness of the wear compensation factor.
Accordingly, it is the object of the present invention to provide an electromagnetically deenergized, spring operated drive unit of the type described in which the retarter acting between the armature and outer body member is eliminated while still providing the wear adjustment feature.